Study of the volatilization phenomena in the process of nuclear waste calcination & vitrification

thesis directors : Dominique THOMAS, Frédéric POINEAU
Ecole doctorale : SIMPPÉ –

jury

The 2-step French process of nuclear waste treatment, which consists in calcining and vitrifying, enable integrating the radio-isotopes into a glassy matrix. The high temperatures involved (above 1000°C) provoke the volatilization of the fission product Cs, in the form of a solid aerosol. Its volatility is increased by the presence of the fission product Tc (simulated by Re). The goal of the work was to characterize the volatilization mechanisms and the retention of the main volatile chemicals in the vitrification process, in order to monitor the volatility from its origin. The first approach was to understand the behavior of the volatile elements in the glass melt, then the transport phenomena to the gas phase, and finally the formation of aerosols. This approach consisted in spiking simplified glass matrices elaborated in a mock-up, with Cs and Re in order to evaluate the effect on volatility. Data treatment was also performed on a series of glass elaborations in a pilot-scale process with diverse elemental concentrations, in order to evaluate the scale-up effects. That led to the conclusion that the Cs-Re volatility comes from the furnace, and to hypothesize about a CsReO4 volatilization mechanism. A second approach was to characterize the evolution of aerosol size distribution throughout the process, and to evaluate the particle capture by a typical dust scrubber. A measurement campaign was performed using an ELPI+ cascade impactor during a glass elaboration on the pilot-scale process. It showed that the aerosol size distribution in the furnace is 1-µm centered. Moreover, MEB-EDS analyses on the particles sampled showed that they are made of solid alkali perrhenates coupled with Mo, that confirm the volatilization mechanism hypothesis. Meanwhile, the characterization of the dust scrubber, with a 1-µm centered simulated alumina aerosol, revealed that the retention is maximized for micronic particles above 2.5 µm.